Vim option and added remove-file functionality

This will allow users to:
```
$ gist --vim my_new_gist.rb
```
Which will create a new file in VIM, then post it as  Gist.

Alternatively:
 ```
$ gist --vim -rf my_new_gist.rb
```
Which will create the new file in VIM, post it as Gist, then immediately remove the file once finished. I chose to do it this way because VIM does not have great support with flushing a buffer to stdout.

This configuration could work with any editor (child process) which will correctly will report it's changed state to the Gist gem. However, I chose to initially set this up with Vim as I am most familiar with it.

Cheers and thanks for the gem :smile: